Signal “Off–OnModel’’for Colorimetric Sensing Proanthocyanidin B₂ Achieved bythe Organic Solvent-Processed Amorphous BiVO₄ Nanozyme

Here, we report a “off–on model”-based colorimetric sensor without the assistance of H₂O₂, achieved by organic solvent-processed amorphous BiVO₄ nanoparticles favorable for large-scale manufacturing. The amorphous material beyond traditional crystalline nanozymes has both vanadium vacancy (<i>V</i>_v) and oxygen vacancy (<i>O</i>_v), as well as single oxidase-like activity with both hydroxyl radical (^•OH) and superoxide anion (O₂^•–) as electron acceptors. Despite the high enzymatic performance, the dual-defect-rich amorphous BiVO₄ preserves good long-term activity in either buffer solution or the solid state. The specially designed BiVO₄ is capable of accelerating TMB oxidation in the presence of polyphenols, thus leading to an interesting signal-intensified colorimetric response. This is probably due to the coordination of the ortho dihydroxyl group in polyphenols with the catalyst by replacing the surface-adsorbed NO₃^–, which results in faster charge transfer between BiVO₄ and substrate and, thus, more rapid depletion of radicals for TMB oxidation. Based on these findings, a sensor platform for proanthocyanidin (PAC) B₂ is established with a limit of detection (LOD) as low as 65 nM and good specificity. The sensor shows higher sensitivity than the standard Porter method (LOD: 0.29 μM) and comparable accuracy when analyzing PAC B₂ in commercially available grape seed capsules, with quantitative recoveries varying from 104.0 to 108.3% and relative standard deviations ranging from 1.7 to 5.7%. To date, this is the only nanozyme-based chemical sensor that is active for the signal “off–on model” for the detection of reducing polyphenols. Also, this method seems quite general, and it can be used for sensing of a series of specific polyphenols in addition to PAC B₂.